Immunosuppressive drugs are commonly used in transplantation and in treatment of autoimmune diseases. Production of these drugs is expensive, and the most frequently used of these drugs, namely cyclosporine A, tacrolimus and rapamycin, exhibit undesirable side-effects. The search for new immunosuppressive drugs devoid of side-effects, particularly in the class of natural peptide immunoregulators and their analogues, represents a serious challenge for medicinal chemistry.
Cyclolinopeptide A (CLA), a very hydrophobic cyclic nonapeptide, was first isolated from linen seeds in 1959. CLA is strongly immunosuppressive, with a potency comparable to that of cyclosporine A (CsA). The mechanism of action of CLA was shown to be similar to that of CsA, i.e. CLA formed a complex with cyclophilin A causing inactivation of calcineurin, albeit at much lower affinity (Gaymes et al., Febs Lett, 1997, 418, 224-227). CLA inhibited both humoral and cellular immune response and graft-versus-host reaction; prolonged survival of allogeneic skin grafts; tempered post-adjuvant polyarthritis in rats and hemolytic anemia of New Zealand Black mice; and, similarly to CsA, inhibited IL1 and IL-2 production. Unfortunately, the high hydrophobicity of CLA presents an obstacle for the potential application of the compound in therapy.
Linear CLA analogues containing alanine residue in successive positions of the peptide chain were found to be immunosuppressive (Wieczorek et al., Arch Immunol Ther Exp, 1992, 40, 213-216). It was also found that the activity of linear CLA analogues gradually decreased with shortening of the peptide chain from the N-terminus, at the same time showing an increase of activity for C-terminal tetra- and tripeptides (Siemion et al., Arch Immunol Ther Exp, 1994, 42, 459-465). The introduction of a single, hydrophilic threonine residue into the CLA molecule did not result in improved solubility in water. However, an improvement in solubility was achieved by the introduction of a sulphonic group in the para-position of the phenyl ring of one or two phenyloalanine residues, without loss of biological activity (Siemion et al., Arch Immunol Ther Exp, 1992, 40, 257-261; Cebrat et al., J Peptide Res., 1997, 49, 415-420). In addition, it has been observed that the inclusion of tetrapeptidic (Pro-Pro-Phe-Phe) or tripeptidic (Pro-Phe-Phe) fragments in longer linear peptides chains seem to have significance for immunosuppressive activity (Wieczorek et al., Arch Immunol Ther Exp, 1993, 41, 291-296; Cebrat et al., Pol. J Chem, 1997, 71, 1401).
A series of analogues in which the cis-peptide bond between proline residues was replaced with 1,5-disubstituted tetrazole ring (a good mimetic of amide bonds in cis configuration) showed immunosuppressive activity comparable to CsA. (Karczmarek et al., Biopolymers, 2002, 63, 343-357).
Synthetic CLA analogues in which leucine residues in position 5 and/or 8 were replaced with their hydroxymethyl analogue displayed a four-fold increase in solubility in water in comparison to CLA, but also showed a 25% diminution in biological activity compared to native CLA (Zubrzak et al., Biopolymers (Peptide Science), 2005, 80, 347-356).
A series of nine CLA analogues was obtained by replacement of CLA proline residues with β2-isoproline and β3-homoproline. In comparison to CsA, these CLA analogues displayed strong inhibitory properties in the cellular immune response. The majority of these analogs were practically devoid of cell toxicity (Katarzyńska et al., J Pept Sci, 2009, 14, 1283-1294).